According to results of recent studies, hydrogen sulfide (H2S) is known to be involved in various physiological phenomena as a 3rd gasotransmitter and gasomediator, together with nitric oxide and carbon monoxide.
A level of hydrogen sulfide in blood plasma is known to be about 50 μM-100 μM, and results of recent studies reported that hydrogen sulfide acts as an important signaling molecule in many different physiological actions such as various inflammatory responses, cardiovascular diseases, vasodilation, glucose metabolism, neovascularization, etc., indicating that it is possible to diagnose a disease such as Down syndrome, Alzheimer's disease, diabetes, liver cirrhosis, etc. simply by detecting the level of hydrogen sulfide.
Particularly, hydrogen sulfide has attracted attention because it functions as a K-ATP channel opener, and contributes to homeostasis of the cardiovascular system, and plays a role in treating cardiac muscle damage. In this regard, WO 2014027820 A1 discloses a real-time biosignal measurement apparatus for cardiac ischemia and reperfusion, including a hydrogen sulfide sensor which detects hydrogen sulfide concentrations in real time.
Further, in vivo hydrogen sulfide is produced by three enzymes of cystathionine-synthase (CBS), cystathionine-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST), and hydrogen sulfide inhibits intracellular oxidative stress by inducing glutathione (GSH) which functions to protect cells by a cell immune system and an antioxidant action. Cytosolic hydrogen sulfide produced by CSE enters mitochondria via Katp channels under conditions of increased intracellular oxidative stress, and induces GSH, together with mitochondrial hydrogen sulfide produced by 3-MST and CAT enzymes, thereby protecting cells.
As such, since hydrogen sulfide induces changes of the GSH concentration to actually protect cells, it serves as a mediator preventing cell dysfunction and inhibiting apoptosis. Therefore, if detection of mitochondrial hydrogen sulfide and measurement of its concentration are possible, it will contribute to studying various life phenomena.
Accordingly, many technologies capable of detecting and quantifying hydrogen sulfide are being developed competitively, and in particular, development of a method of detecting in vivo hydrogen sulfide by non-invasive imaging has emerged as an important issue.
Although involvement of hydrogen sulfide in disease diagnosis and physiological phenomena has received attention and various types of probes are now being introduced, detection and accurate quantification of hydrogen sulfide are known to require very difficult and complex conditions such as measurement of H2S in a liquid state, selectivity for hydrogen sulfide over other anionic species, selectivity for hydrogen sulfide over reduced glutathione (GSH), etc.
In this regard, a method of detecting hydrogen sulfide by using a chromophore that utilizes chemical properties of intracellular cytosolic hydrogen sulfide and blood plasma hydrogen sulfide, a method of detecting hydrogen sulfide by passing a specific electrode through sulfide ions, and a method of detecting hydrogen sulfide by gas chromatography have been used until now. In recent years, various fluorescent probes have been developed. However, due to limitations of the fluorescent probe method, detection of hydrogen sulfide through imaging is very limited to a small animal level, and its application in practical bioimaging and studies on life phenomena is extremely restricted.
As a general fluorescent probe for detecting H2S, a 2,4,6-triaryl pyridium cation compound was disclosed (Journal of the American Chemical Society, 125, 9000, 2003), but there is a problem that this compound cannot avoid competitive reaction with GSH. Further, a method of using 2,4 dinitrobenzene sulfonyl fluorescein as a fluorescence enhancement probe was suggested (Analytica Chimica Acta, 631, 91, 2009), but there is a problem that fluorescence intensity is changed over time due to hydrolysis of sulfonic acid ester.
In addition, an aminofluorescein compound (DPA-4-AF) having 2,2-dipicolylamine containing divalent copper ions (Cu2+) was disclosed (Myung Gil Choi, et. al., Chem. Commun., (47), 7390-7392, 2009). A compound, in which a fluorescent material is attached to cyclen, cyclam, TACN, etc., is disclosed in WO2012-144654A1. This compound itself emits fluorescence, but it does not emit fluorescence by quenching of copper ions once binding with copper to form a complex compound. When hydrogen sulfide reacts with the complex compound, the copper complex is detached in a CuS form. Depending on the degree, fluorescence intensity to be restored differs. On the basis of this principle, a method of quantifying the amount of hydrogen sulfide was disclosed. However, this compound still has many problems in that images can be acquired only in vitro, and it is difficult to selectively detect hydrogen sulfide in vivo. Development of a new technology capable of replacing the known detection methods is still demanded.